Seeds started indoors usually germinate, but the resulting seedlings are sometimes weak from lack of light and perform poorly when placed in the garden. Gardeners wanting only a few plants are probably better off purchasing them from a local garden center.
Damping-off is a fungus disease common on seedlings started at home. The disease causes seedling stems to shrivel and turn brown at the soil line. Infected plants fall over. At first, only a few fallen plants are seen then more, until practically all the seedlings are dead. Damping off is more of a problem on overwatered plants growing in poorly drained soils.
Using pasteurized soil is another way gardeners avoid diseases. Fill a pan or metal tray with a 4 inch, or less, layer of moist soil. Bury a one and a half inch diameter potato in the center of the soil, then cover the pan with aluminum foil, and seal the edges. Punch a small hole in the center of the aluminum foil and insert the bulb end of a candy thermometer. Place the pan in an oven at 180 to 200 degrees. After the thermometer reads 180 degrees, leave the soil in the oven 30 minutes then remove and allow it to cool. The potato will be cooked if the soil is properly pasteurized. Pasteurizing the soil is useless if dirty containers are used. Pasteurizing soil in the oven can create an odor some people find offensive. Clean clay pots can be sterilized by baking while the soil is heated. Pots can be sterilized by soaking them in a solution of one part chlorine bleach and nine parts water. Rinse, and allow the pots to dry thoroughly before filling them with soil.
An alternative is the purchase of new, plastic or fiber, disposable trays or flats. Plants grown in peat pots suffer little setback when transplanted into the garden.
Use a loose, well-drained, fine textured soil mix that is low in nutrients. A suitable mix is equal parts of pasteurized garden soil, sand and sphagnum peat moss. Commercially prepared mixtures may also be used.
Fill the containers about two thirds full with soil. Level the soil and soak it thoroughly. Sift more soil mixture through window screening to form a layer that fills one fourth to one half of the remaining depth of the container.
Make a furrow one fourth of an inch deep in the sifted soil. Sow large seeded plants directly into the bottom of the furrow. Before sowing small seed, fill the furrow with vermiculite, then sow small seed on the surface of the vermiculite.
Sow seed in flats at the rate recommended on the seed packet. Sow two to four seeds per peat pot if the seeds are large. After planting, cover the furrows with a thin layer of vermiculite, then mist with water. A fine mist prevents washing the seed out of the soil. Some seed should not be covered. To be sure, check the cultural suggestions for each plant before sowing the seed.
Once seeds are planted, place a sheet of plastic over the containers and provide temperatures between 70 and 75 degrees. The containers need no more water until the seed germinates. Under no circumstances should plastic covered containers be placed in direct sunlight.
Once seedlings are growing, remove the plastic and provide proper growing conditions. Give the seedlings adequate light. Even the sunniest windowsill provides varying amounts of light and only from one direction. Windowsills often lack adequate humidity and are too warm for best seedling growth. Cool white fluorescent lights placed three to six inches above the seedlings are a good light source. The length of time the lights are on varies with the type of annual grown. Some general guidelines are given here, but read the seed packet for additional information. A time clock will make sure the daylengths are regular. If only one light fixture and time clock are used, the plants must have compatible growth requirements.
These plants need 10 to 12 hour days and temperatures between 60 and 65 degrees; calliopsis, china aster, cornflower, gaillardia, petunia, phlox, poppy, rudbeckia, salpiglossis, scabiosa, snapdragon, and verbena.
Most other plants need 18 hour days. On short days they form flowers and never produce good flowering plants outdoors. These plants are grown under 18 hour days at 65 degrees; cockscomb, cosmos, dahlia, marigold, morning glory, scarlet sage, sunflower, and zinnia.
Annuals not included in the above groups are grown under 18 to 20 hour days.
After removal of the plastic, the seedlings must be watered frequently and fertilized. Water when the soil surface begins to dry and use a house plant fertilizer according to label directions.
Seedlings in flats should be transplanted to other containers at wider spacings once two true leaves have developed. Handle the seedlings very carefully as they are easily injured. Use different flats but the same sterile soil mix. Use a spacing of one and a half inches between plants.